Palladium is widely used in various materials such as dental crowns, catalysts, fuel cells and jewelry. The application of palladium as an automobile catalytic converter has evidently controlled the pollution of vehicle exhaust, but at the same time, a significant quantity of palladium deposited in the roadside soil (<0.3 ppm) and plants. Rain may wash them into river, lake and sea which would lead to the pollution of water system. Additionally, palladium is commonly employed as an efficient catalyst for the synthesis of complex molecules, therefore it plays a very important role in pharmaceutical industry. Despite the wide application in catalytic reaction, a high level of residual palladium is often found in the resultant product (typically 300 to 2000 ppm) and in the reactor, which thus may be a health hazard. For example, palladium content in drugs is limited to 5 to 10 ppm. Therefore, a convenient, fast, highly sensitive and selective detection method for palladium is urgently needed.
Traditional detection methods for palladium, such as atomic absorption spectrometry, plasma emission spectrometry, solid phase microextraction-high performance liquid chromatography and x-ray fluorescence, usually suffer from high cost due to complicated pretreatment process for the sample and operation by highly trained individuals that limited their application. Fluorescence probe has attracted much attention due to high sensitivity, good selectivity, fast response and capability for the visual detection. As an open-shell transition-metal ion, Pd2+ displays an evident fluorescence quenching. Hence most of probes for Pd2+ detection are designed by colorimetric and fluorescence-quenched methods. There are only threefluorescence-enhanced Pd2+ probes reported. Generally, compared with the detection for quenched fluorescence, the detection for increased fluorescence is more reliable, lower in detection limit and better in properties. So how to avoid fluorescence-quenching of Pd2+ is crucial to Pd2+ probe. Here we synthesized a rhodamine-based fluorescence-enhanced probe based on triphenylphosphine ligand coordinating Pd2+ for the first time, which can provide us both fluorometric and colorimetric methods for visual detection of Pd2+.
There have been three examples of fluorescence-enhanced probes specific for Pd2+ were reported: the first one is designed based on thioether-maleonitrile ligand as receptor; the second one is designed based on Tsuji-Trost allylic oxidation insertion reaction; the third one is the rhodamine probe designed by our research team based on allyl group coordinating Pd2+. They all display their own disadvantages: the first thioether-maleonitrile fluorescence probe displays a fluorescence spectrum change after coordination with Pd2+, but excitation wavelength thereof is not in visual light area and can not be used for Pd0 detection as well (Thomas Schwarze, Holger Muller, Carsten Dosche, Tillmann Klamroth, Wulfhard Mickler, Alexandra Kelling, Hans-Gerd Lohmannsroben, Peter Saalfrank and Hans-Jurgen Holdt, Angew. Chem. Int. Ed., 2007, 46, 1671-1674); the second allylic oxidation insertion reaction-based fluorescence probe can not avoid the interference from Pt2+ (Fengling Song, Amanda L. Garner and Kazunori Koide, J. Am. Chem. Soc., 2007, 129(41), 12354-12355); although the third allyl coordinating Pd2+-based probe has solved the problems of the above-mentioned two probes, it still displays a disadvantage of a long balance time (Honglin Li, Jiangli Fan, Jianjun Du, Kexin Guo, Shiguo Sun, Xiaojian Liu and Xiaojun Peng, Chem. Commun., 2010, DOI: 10. 1039/b916915f).